A while back I made a rough summary of what's been added in recent versions:
2.10: security camera overhaul, xrays expanded to AIs, official support for secrets, new furniture, subtitles
2.09: animated prefab of the steam engine, new assets (furniture, fireflies, ambient AIs, blue fire elementals, Eve statue, naval clock)
2.08: xrays, new monsters (beastmen, running zombies, rudimentary werebeast, flamebearing skeletons), new assets (body bags, scientific instruments/papers, covered furniture, various Dragofer's stuff including decorative ship models)
2.07: more Springheel modules, a few new assets